The field is internal combustion engines, particularly two-stroke opposed-piston engines. In one aspect, the field relates to cooling opposed pistons with contoured end surfaces. In another aspect, the field relates to articulation of opposed pistons with crankshaft linkages.
As seen in FIG. 1, an internal combustion engine is illustrated by way of an opposed-piston engine that includes at least one cylinder 10 with a bore 12 and longitudinally-displaced exhaust and intake ports 14 and 16 machined or formed therein. Fuel injector nozzles 17 are located in or adjacent injector ports that open through the side of the cylinder, at or near the longitudinal center of the cylinder. Two pistons 20, 22 are disposed in the bore 12 with their end surfaces 20e, 22e in opposition to each other. For convenience, the piston 20 is referred as the “exhaust” piston because of its proximity to the exhaust port 14; and, the end of the cylinder wherein the exhaust port is formed is referred to as the “exhaust end”. Similarly, the piston 22 is referred as the “intake” piston because of its proximity to the intake port 16, and the corresponding end of the cylinder is the “intake end”.
Operation of an opposed-piston engine with one or more cylinders such as the cylinder 10 is well understood. In this regard, and with reference to FIG. 2, in response to combustion occurring between the end surfaces 20e, 22e the opposed pistons move away from respective top dead center (TDC) positions where they are at their closest positions relative to one another in the cylinder. While moving from TDC, the pistons keep their associated ports closed until they approach respective bottom dead center (BDC) positions in which they are furthest apart from each other. The pistons may move in phase so that the exhaust and intake ports 14, 16 open and close in unison. Alternatively, one piston may lead the other in phase, in which case the intake and exhaust ports have different opening and closing times.
In many opposed-piston constructions, a phase offset is introduced into the piston movements. As shown in FIG. 1, for example, the exhaust piston leads the intake piston and the phase offset causes the pistons to move around their BDC positions in a sequence in which the exhaust port 14 opens as the exhaust piston 20 moves through BDC while the intake port 16 is still closed so that combustion gasses start to flow out of the exhaust port 14. As the pistons continue moving away from each other, the intake port 16 opens while the exhaust port 14 is still open and a charge of pressurized air (“charge air”) is forced into the cylinder 10, driving exhaust gasses out of the exhaust port 14. The displacement of exhaust gas from the cylinder through the exhaust port while admitting charge air through the intake port is referred to as “scavenging”. Because the charge air entering the cylinder flows in the same direction as the outflow of exhaust gas (toward the exhaust port), the scavenging process is referred to as “uniflow scavenging”.
As the pistons move through their BDC locations and reverse direction, the exhaust port 14 is closed by the exhaust piston 20 and scavenging ceases. The intake port 16 remains open while the intake piston 22 continues to move away from BDC. As the pistons continue moving toward TDC (FIG. 2), the intake port 16 is closed and the charge air in the cylinder is compressed between the end surfaces 20e and 22e. Typically, the charge air is swirled as it passes through the intake port 16 to promote scavenging while the ports are open and, after the ports close, to mix the air with the injected fuel. Typically, the fuel is diesel which is injected into the cylinder by high pressure injectors. With reference to FIG. 1 as an example, the swirling air (or simply, “swirl”) 30 has a generally helical motion that forms a vortex in the bore which circulates around the longitudinal axis of the cylinder. As best seen in FIG. 2, as the pistons advance toward their respective TDC locations in the cylinder bore, fuel 40 is injected through the nozzles 17 directly into the swirling charge air 30 in the bore 12, between the end surfaces 20e, 22e of the pistons. The swirling mixture of charge air and fuel is compressed in a combustion chamber 32 defined between the end surfaces 20e and 22e when the pistons 20 and 22 are near their respective TDC locations. When the mixture reaches an ignition temperature, the fuel ignites in the combustion chamber, driving the pistons apart toward their respective BDC locations
In some aspects of two-stroke, opposed-piston engine construction it is desirable to utilize pistons with crowns having highly contoured end surfaces that interact with swirl and with squish flow from the periphery of the combustion chamber to produce complex, turbulent charge air motion that encourages uniform mixing of air and fuel. However, combustion imposes a heavy thermal load on the piston crowns. The highly contoured end surfaces create non-uniform thermal profiles that are not suitably cooled by conventional forced cooling configurations, leading to asymmetrical thermal stress, wear, and piston crown fracture. It is therefore desirable to provide piston constructions with the capability of cooling the contoured crowns of such pistons.
In other aspects of two-stroke, opposed-piston engine construction, the nature of the operating cycle results in the uninterrupted application of compressive loads on bearings in the piston-to-crankshaft connecting linkages. These non-reversing loads prevent the separation of bearing surfaces, which blocks the flow of lubricant therebetween and limits the durability of such bearings. As a result, during opposed-piston engine operation a continuous compressive load is exerted on wristpin bearings throughout the operating cycle. It is therefore desirable to provide piston constructions with the capability of articulating the piston with a connecting rod by way of a bearing construction that separates bearing surfaces for provision of lubrication during the operational cycle of an opposed-piston engine.
In yet other aspects of two-stroke, opposed-piston engine construction, the cooling and bearing structures are both housed in a limited space defined by the piston skirt. It is therefore desirable to provide a piston construction that positions those structures with a minimum of occupied space.